Process of producing fiber of high alpha cellulose content



Patented Aug. 26, 1930 UNITED STATES PATENT OFFICE GEORGE A. RICHTER, 0F BERLIN, NEW HAMPSHIRE, ASSIGNOR TO BROWN COMPANY,

' OFEBERLIN, NEW HAMPSHIRE, A CORPORATION OF MAINE PROCESS OF PRODUCING FIBER OF HIGH ALPHA CELLULOSE CONTENT No Drawing.

This invention relates to the production of fiber of high alpha cellulose content from raw or unbleached fiber, such as sulphite, kraft, or soda pulp, and has for its object to produce fiber of high alpha cellulose content suitable for manufacture into high grade writing papers and for conversion into cellulose derivatives of good quality.

In application, Serial No. 716,154, filed May 27, 1924, by George A. Richter. and Milton 0. Schur,,there isdesoribed a process of producing fiber of high alpha cellulose content. The process usually comprises first treating the raw fiber with chlorine or bleach solution, then digesting such fiber in an alkaline solution, e. g., a caustic soda solution, and finally bleaching the digested fiber to the desired whiteness.

Such a process is satisfactory when a normally cooked raw sulphite fiber, for example, is employed as a raw material, but when kraft pulp or a materially undercooked sulphite pulp is employed, a larger quantity of bleach or chlorine then ordinarily is necessary for initially treating the fiber, in order to produce a final fiber of satisfactory color without the use of excess bleach or chlorine in the final bleaching step. The use of a large quantity of bleach or chlorine is liable so to weaken the fiber by causing excessive degradation into bxycelluloses.

I have found that weakening of kraft, undercooked sulphite, or other raw fiber may be avoided in producing alpha fiber. This is accomplished by digesting the raw fiber, (only slightly pretreated with bleach or chlorine solution, if desired) in a liquor containing an oxidizing agent, such as bleach, sodium peroxide or permanganate, or other suitable oxidant and a substantial proportion of an alkali, such as caustic soda. T-he fiber may first be digested at about room temperature until the oxidant is largely consumed, and then digested in the same liquor, which contains a material proportion of caustic soda, at an elevated temperature.

During the initial digestion, the caustic I soda protects the alpha or resistant cellulose from degradation into oxycelluloses, by retarding the oxldizing action to the desired Application filed September 16, 1926. Serial No. 135,995.

intensity, and permitting a selective removal of substantially only ligneous and resinous matter to take place. The caustic soda undergoes little reaction with the cellulose fibers or other components of the pulp at room temperature,analysis after the lowtemperature digestion showing that the proportion of free caustic soda is substantially the same as in the initial liquor. Upon elevating the temperature of digestion, however, the caustic soda reacts with and dissolves unstable celluloses from the fiber, and also some of the reaction products resulting from the oxidation with bleach.

A procedure which may be practised in. producing a product containing a high percentage of alpha cellulose from sulphite pulp, for example, may be substantially as follows. The pulp is first preferably treated with a chlorine solution containing 1% to 2% chlorine, and then washed. The treated pulp is then digested in the form of a 10% suspension in a liquor containing about from 4% to 8% calcium bleach and 6% to 8% caustic soda, based on dry pulp. The suspension is maintained at room temperature while being agitated, until the bleach has been largely consumed by reaction with the resinous and ligneous components of the pulp. The suspension is'then elevated to a temperature of, say, 210 to 212 F., and maintained at such temperature for four to six hours, thereby eifecting-aremoval of less-resistant celluloses and also certain reaction products resulting from the reaction with bleach. The resultant fiber is light colored, and requires only a mild bleaching action to whiten it to its maximum color. The length of fiber is substantisodium peroxide is employed, less caustic soda is necessary than in the example hereinbefore given, as the sodium peroxide decomposes to yield caustic soda. The caustic soda content of the initial liquor is approximately that necessary for digestion at elevated temperature minus approximately the sodium hydroxide formed by decomposition of the sodium peroxide during the oxidizing action. Thus, the digesting liquor may contain about 6% to 7% sodium peroxide and about 2% to 3% caustic soda.

If the preliminary treatment with chlorine solution is omitted, the amount of bleach employed with the caustic soda in the digesting liquor may be increased to intensify the oxidizing action somewhat. Either a sodium or calcium base bleach may be employed in the digesting liquor. When the digesting liquor is made up of calcium bleach and caustic soda, as in the example given, a precipitation of calcium hydroxide and a formation of sodium bleach takes place, so that the alkali component includes both caustic lime and caustic soda. The liquor may also be made up with sodium bleach and caustic soda, or with sodium bleach and lime.

In certain cases it may be advantageous I to shorten the low-temperature digesting period, or to carry out the entire digestion at an elevated temperature,-say, 210 F. Under such condition, a lower yield of fiber is obtained, but such fiber is of lower viscosity* and may be more satisfactory for certain purposes.

lVhen a fairly well cooked sulphite pulp is used as a raw material, a white product of high alpha cellulose content may be produced without a final bleaching step. Digestion may be carried out, as for instance in a four-tank continuous system. The raw fiber, made up in the form of a 10% stock suspension in a digesting liquor containing about 6% calcium bleach and 7% caustic soda, based on fiber, is agitated and maintained at about room temperature in the first tank, and then passed through the other tanks while being agitated and maintained at about boiling temperature,complete digestion requiring from four to six hours. The product leaving the last tank is washed thoroughly, and is finally slightly acidified, as with sulphurous acid. A fiber thus produced possesses good strength and excellent tear resistance.

Kraft fiber may be treated similarly to The term viscosity. as applied to cellulose fiber, is an arbitrary one, descriptive of the viscosity of solution,-- cuprammonium cellulose solution, for example, which may be prepared therefrom.

stock suspension in a liquor containing 10% to 15% bleach and 6% to 10% caustic soda, based on pulp. The suspension is agitated and maintained at room temperature for about three hours, to permit the oxidizing reaction to take place, at the end of which period the temperature is elevated to about boiling (210214 F.) and maintained for fiveto six hours, to permit the caustic soda to react with the less-resistant celluloses and certain reaction products of oxidation. As a result of such digestion, the tear resistance of a paper made of such fiber is mater ally increased, while its strength is substantially unaffected. The digested kraft pulp 1S bleachable to a pure white color with a moderate amount of bleach. If undercooked kraft fiber is employed as the raw material, it may be desirable preliminarily to treat such fiber with a 1% to 2% chlorine solution.

Soda pulp or fiber produced by other fiberliberating operations may be similarly treated, the specific conditions of treatment employed for a particular fiber dependmg upon its characteristics and'the characteristics of the product which it is desired to produce.

I do not herein claim the process disclosed in my application Serial No. 75,888, filed December 16, 1925, which comprises digesting a raw pulp, e. g., sulphite pulp, in an alkaline liquor, bleaching such digested pulp, and re-digesting the bleached pulp in an alkaline liquor.

Having thus described this invention, it is evident that various chan es might be resorted to without departing from its spirit or scope as defined by the appended claims.

What I claim is:

1. A process of producing fiber of high alpha cellulose content from a raw pulp, which comprises digesting the raw pulp at relatively low temperature in an alkaline liquor under oxidizing conditions, and then digesting the pulp at an elevated temperature in the same liquor.

2. A process of producing fiber of high alpha cellulose content from a raw pulp, which comprises digesting the raw pulp at relatively low temperature in a liquor containing an alkali and an oxidizing agent, and then digesting the pulp at an elevated temperature in the same liquor.

3. A process of producing fiber of high alpha cellulose content from a raw pulp, which comprises digesting the raw pulp at relativel low temperature in a liquor containing bleach and an alkali, and then digesting the pulp at an elevated temperature in the same liquor.

4. A process of producing fiber of high alpha cellulose content from a raw pulp, which comprises digesting the raw pulp at relatively low temperature in a liquor containing bleach and an alkali, digesting the ing bleach and also a substantial amount of alkali sutficient to remove unstable celluloses and reaction products produced by the bleach and to prevent degradation of the alpha fiber. I

6. A process of producing fiber of high I alpha cellulose content from raw pulp, which comprises digesting the raw pulp in a liquor containing bleach and an alkali until the bleach is largely consumed by reaction with resinous and ligneous matter, and then digesting the pulp in the same liquor at an elevated temperature to dissolve unstable cellulose's therefrom.

7. A process of producing fiber of high alpha cellulose content from unbleached pulp, which comprises treating the unbleached pulp with a weak oxidizing reagent, digesting the treated pulp at rela tively low temperature in a liquor containing bleach and an alkali until the bleach is largely consumed, and then digesting the pulp in the same liquor at an elevated temperature to dissolve unstable celluloses therefrom.

8. A process of producing fiber of high alpha cellulose content from a raw pulp, which comprises digesting the raw pulp at about room temperature ina liquor containing bleach and caustic soda, and then digesting the pulp in the same liquor at elevated temperature.

9. A process of producing fiber of high alpha cellulose content from a raw pulp, which comprises digesting the raw pulp at relatively low temperature in a liquor containing alkali and an oxidant, and then digesting the pulp at elevated temperature in an alkaline liquor.

10. A process of producing fiber of high alpha cellulose content from a raw pulp, which comprises digesting the raw pulp at relatively low temperature in a liquor containing an alkali and an oxidant, digesting the pulp at elevated temperature in an alkaline liquor, and bleaching the digested pulp.

11. A process of producing fiber of high alpha cellulose content from a raw pulp, which comprises treating the raw pulp with a relatively weak oxidizing reagent, digesting the treated pulp at relatively low temperature in a liquor containing alkali and an oxidant, digesting the pulp at elevated temperature in an alkaline liquor, and bleaching the digested pulp.

In testimony whereof I have afixed my signature. p

GEORGE A. RICHTER. 

